QTL mapping for downy mildew resistance in cucumber inbred line WI7120 (PI 330628)
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Host resistance in WI7120 cucumber to prevailing downy mildew pathogen field populations is conferred by two major-effect, one moderate-effect and two minor-effect QTL.
Downy mildew (DM) caused by the obligate oomycete Pseudoperonospora cubensis is the most devastating fungal disease of cucumber worldwide. The molecular mechanism of DM resistance in cucumber is poorly understood, and use of marker-assisted breeding for DM resistance is not widely available. Here, we reported QTL mapping results for DM resistance with 243 F2:3 families from the cross between DM-resistant inbred line WI7120 (PI 330628) and susceptible ‘9930’. A linkage map was developed with 348 SSR and SNP markers. Phenotyping of DM inoculation responses were conducted in four field trails in 2 years at three locations. Four QTL, dm2.1, dm4.1, dm5.1, and dm6.1 were consistently and reliably detected across at least three of the four environments which together could explain 62–76 % phenotypic variations (R 2). Among them, dm4.1 and dm5.1 were major-effect QTL (R 2 = 15–30 %) with only additive effects; dm2.1 (R 2 = 5–15 %) and dm6.1 (R 2 = 4–8 %) had moderate and minor effects, respectively. Epistatic effects were detected for dm2.1 and dm6.1 with both dm4.1 and dm5.1. One additional minor-effect QTL, dm6.2 (R 2 = 3–5 %) was only detectable with the chlorosis rating criterion. All alleles contributing to DM resistance were from WI7120. This study revealed two novel QTL for DM resistance and the unique genetic architecture of DM resistance in WI7120 conferring high level resistance to prevailing DM populations in multiple countries. The effects of disease rating scales, rating time and criteria, population size in phenotyping DM resistance on the power of QTL detection, and the use of DM resistance in WI7120 in cucumber breeding were discussed.
KeywordsDowny Mildew Plant Introduction Disease Score Downy Mildew Resistance Cucumber Line
The authors thank Kristin Haider for technical help. We are also indebted to Vittorio Stravato, and Giuseppe Carannante for help phenotyping of DM resistance in the IT2013 experiment. This research was supported by the US Department of Agriculture (USDA)-Specialty Crop Research Initiative Grant (SCRI, project # 2011-51181-30661) to YW. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. USDA is an equal opportunity provider and employer.
Compliance with ethical standards
Conflict of interest
The authors declares no conflict of interest.
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